linux-kernel - Re: [PATCH RFC] x86: avoid atomic operation in test_and_set_bit

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Message-ID: <AANLkTinpzu5JqHBKkH7DMpWqoNu1rBMTNXqEVph+6XX0@mail.gmail.com>
Date:	Thu, 24 Mar 2011 10:01:24 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Nikanth Karthikesan <knikanth@...e.de>
Cc:	Ingo Molnar <mingo@...hat.com>, Nick Piggin <npiggin@...nel.dk>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Beulich <JBeulich@...ell.com>,
	Jack Steiner <steiner@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] x86: avoid atomic operation in test_and_set_bit_lock
 if possible

On Wed, Mar 23, 2011 at 9:56 PM, Nikanth Karthikesan <knikanth@...e.de> wrote:
> On x86_64 SMP with lots of CPU atomic instructions which assert the LOCK #
> signal can stall other CPUs. And as the number of cores increase this penalty
> scales proportionately. So it is best to try and avoid atomic instructions
> wherever possible. test_and_set_bit_lock() can avoid using LOCK_PREFIX if it
> finds the bit set already.

This is potentially _very_ wrong. It means that test_and_set_bit() is
no longer a serializing instruction in the failure case, and I wonder
what effect that will have on the thousands of users.

It also means that test_and_set_bit() on an uncached entry now starts
out with a read-for-ownership cache operation, which can be quite a
bit slower than the exclusive ownership thing for the hopefully common
case where it succeeds.

So no, I really think this is seriously wrong. It basically makes it
impossible for the user of the bitop function to do a good job if it
wants to.

WHICH test_and_set_bit() are you having performance issues with?
Because I think the right approach is to do this optimization on a
case-by-case basis in the code that actually does the operation, not
in the low-level routine.

                       Linus
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